The invention relates to new double metal cyanide (DMC) catalysts, to a process for the preparation of these new double metal cyanide catalysts, to a process for the preparation of polyetherpolyols by the polyaddition of alkylene oxides to starter compounds which contain active hydrogen atoms in the presence of these new double metal cyanide catalysts, and to the polyetherpolyols produced by this process.
Double metal cyanide (DMC) catalysts for the polyaddition of alkylene oxides to starter compounds containing active hydrogen atoms are known and described in, for example, U.S. Pat. Nos. 3,404,109, 3,829,505, 3,941,849 and 5,158,922. The use of these DMC catalysts for preparing polyetherpolyols causes, in particular, a reduction in the proportion of monofunctional polyethers with terminal double bonds, i.e., the so-called monools, as compared with the conventional preparation of polyetherpolyols using conventional alkali metal catalysts such as, for example, alkali metal hydroxides. The polyetherpolyols obtained in this way may be processed to produce high quality polyurethanes (e.g., elastomers, foams, coatings).
DMC catalysts are, in general, usually obtained by reacting an aqueous solution of a metal salt with an aqueous solution of a metal cyanide salt in the presence of an organic complex ligand such as, for example, an ether. In a typical catalyst preparation, for example, aqueous solutions of zinc chloride (in excess) and potassium hexacyanocobaltate are mixed and then dimethoxyethane (glyme) is added to the suspension produced. After filtering and washing the catalyst with an aqueous glyme solution, an active catalyst of the general formula: EQU Zn.sub.3 [Co(CN).sub.6 ].sub.2.xZnCl.sub.2.yH.sub.2 O.zglyme
is obtained (see, for example, EP-A 700 949).
Other DMC catalysts are disclosed in, for example, JP-A 4 145 123, U.S. Pat. No. 5,470,813, EP-A 700 949, EP-743 093, EP-A 761 708 and WO 97/40086, which are described as further reducing the proportion of monofunctional polyethers with terminal double bonds during the preparation of polyetherpolyols by using tertiary butanol as the organic complex ligand. Tertiary butanol can be used either alone, or combined with a polyether (see, for example, EP-A 700 949, EP-A 761 708, and WO 97/40086). In addition, the induction time during the polyaddition reaction of alkylene oxides with corresponding starter compounds is reduced and the catalyst activity is increased by the use of these DMC catalysts.
The object of the present invention was to provide improved DMC catalysts for the polyaddition of alkylene oxides to corresponding starter compounds which exhibit additionally increased catalytic activity as compared with the currently known catalyst types. This leads to improved economic viability of the method for the preparation of polyetherpolyols due to the shortened alkoxylation times. Ideally, as a result of this increased catalytic activity, the catalyst can then be used in such small concentrations (i.e., 25 ppm or less) that the costly procedure required to separate the catalyst from the product is no longer necessary and the resultant polyetherpolyol product can be used directly for polyurethane production.
Surprisingly, it has now been found that DMC catalysts which contain a cyclodextrin as an additional complex ligand exhibit greatly increased activity during the process for the production of polyetherpolyols.